This invention relates to the fabrication of semiconductor integrated circuits. In particular, this invention is directed to a method of simultaneously forming electrically isolated areas and conductive contact areas in semiconductor material.
When fabricating integrated circuits, and particularly highly dense dielectrically isolated devices, narrow trenches are formed to provide isolation. Also points where substrate contact is desired must be established in very small zones. It is known that undoped polysilicon and epitaxial silicon each have very high electrical resistance. Thus, a defect in a trench sidewall or at the passivation layer on the top of a silicon trench fill will not impact the characteristics of electrical circuits if a second defect is positioned a few .mu.m away from the first defect. Consequently, the use of undoped polysilicon trench isolation as used typically in known RAM chips is considered a very low risk isolation scheme. That is, so long as defects are disposed far enough away, the presence of an electrical short giving rise to a particular trench sidewall defect will not be detrimental to operation of that device.
The use of undoped polysilicon trench isolation also does not require the step of passivation layer removal at the trench bottom. Consequently, the trench sidewall is not exposed to etching operations such as reactive ion etching (RIE) and, therefore, does not need additional protective layers such as Si.sub.3 N.sub.4 which also cause additional problems in terms of forming dislocations near trenches in subsequent heat processing steps.
While the use of undoped polysilicon trench isolation offers certain advantages, it is considered disadvantageous since there is no easy technique within that methodology for forming a silicon substrate contact. If, for example, doped polysilicon or epitaxially grown silicon is employed, the presence of one defect will cause device and circuit failure. Within the art, there is no known technique for providing a doped silicon trench area, needed to form a substrate contact in device processing, yet within the same process utilize undoped polysilicon for purposes of trench isolation.
The prior art is replete with a number of examples which illustrate the use of polysilicon as a trench filling material. Reference is made to U.S. Pat. Nos. 4,473,598; 4,140,558 and 4,252,579. Various silicon trench filling techniques to achieve isolation are also disclosed U.S. Pat. No. 4,526,631, and in IBM Technical Disclosure Bulletin Nos.: Vol. 25, No. 2, p. 558, July 1982; Vol. 25, No. 6, p. 3105, November 1982; Vol. 27, No. 2, p. 1313 et seq., July 1984; and Vol. 27, No. 3, p. 1481, Aug. 1984. While the prior art, as exemplified by these references, provides a number of solutions of providing a silicon trench fill scheme, none provide for the simultaneous formation of doped and undoped epitaxial silicon fill. Some prior art devices employ undoped silicon fill for device trench isolation but are encumbered with the problems of providing substrate contact in other areas where a doped silicon material is required. In those techniques where doped silicon material is used throughout, the problem of complete device failure given proximity of defects is unsolved. Moreover, in some devices there exists no simple process for forming the necessary silicon substrate contact. In some known devices, the wafer back cannot be employed as a silicon substrate contact given the presence of ancillary structure. Such is found in thermal conduction modules wherein the back side of the wafer is used for purposes of heat dissipation and the like.